Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS), referred to as Node Bs with regard to a universal mobile telecommunication system (UMTS), and a plurality of subscriber units, often referred to as user equipment (UE) in UMTS.
In a cellular network, such as UMTS, the power transmitted by a UE is regulated in order to minimise interference with other UEs. Typically, the output power generated by the radio frequency (RF) power amplifier (PA) in the UE will vary due to any number, or combination, of factors, such as the manufacturing process, operating temperature, supply voltage, antenna loading and other such factors. Thus, it often becomes necessary to measure the radio frequency transmit power at, or after, the PA output and to control a PA gain, typically by controlling the gain of amplifiers located earlier in the amplifier chain, in response to this measurement. This feedback will allow power control regulation to compensate for variations in PA supply voltage, operating temperature & manufacturing process.
Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear radio frequency power amplifier. Thus, it is important to ensure any unwanted terms arising from the intermodulation are minimised and stay below a specified value. By modifying the bias to improve efficiency the PA becomes more non-linear, which in turn would lead to increased intermodulation problems. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions. It is also known that non-linearities may also create in-band distortion, which is typically measured by determining an error vector magnitude (EVM). Quantum processes within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, the low efficiency is also recognised as being problematic for the base stations.
Furthermore, the emphasis in portable (subscriber) equipment is to increase battery life. The emphasis for base station designers is to reduce operating and equipment cost (power consumption, size, power dissipation, etc.). Hence, such operating efficiencies of the amplifiers used must be maximised. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number of linearising techniques exist, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion (APD) and these are often used to resolve the inherent trade off of linearity versus efficiency in wireless communication units.
To fully exploit the advantages of APD, a system that will automatically adjust the power amplifier (PA) supply is also required, so that it may be optimally biased across, say, the 3GPP power target range as well as in the presence of process, temperature and/or load variation. It is known that without such a power tracking or Envelope Tracking (ET) system the PA can become over-compressed, resulting in a poor adjacent channel leakage power ratio (ACLR) and/or error vector magnitude (EVM) failure. Alternatively, the PA may be under-compressed resulting in reduced efficiency performance.
Known techniques using APD have been targeted at base station applications where the requirements on optimally biasing the PA across load and power target variation are less severe compared to a handset wireless communication design.
Furthermore, known techniques that use ET are invariably analogue-based, which is somewhat incompatible with the trend towards digital-based solutions, as used in APD. For example, the document titled ‘A High-Efficiency Linear RF Power Amplifier with a Power-Tracking Dynamically Adaptive Buck-Boost Supply’ authored by Sahu et al., and published in IEEE Trans MTT vol. 52, no 1 Jan. 2004 and the document titled ‘High Efficiency Power Amplifier Using Dynamic Power-Supply Voltage for CDMA Applications’, authored by Hannington et al., and published in IEEE Trans on MTT vol. 47 no 8 Aug. 2000 are typical examples of an analogue-RF precision solution to power tracking.